Thai stocks slumped 4.69 percent Thursday amid concerns over King Bhumibol Adulyadej's health, and despite assurances from the palace that the 81-year-old's condition was "good".

The Stock Exchange of Thailand (SET) was down by 4.69 percent to 697.13 points on rumours that the health of the 81-year-old was deteriorating, despite assurances from the palace Wednesday night that his condition was "good".

King Bhumibol, the world's longest-serving monarch, was admitted to hospital on September 19 and put on a drip after being diagnosed with a lung infection, fever, fatigue and loss of appetite.

Rumours of his ill-health circulated globally Wednesday, causing a 2.04 percent drop in Thai stocks by the close of trading.

"Foreign investors continue to sell blue-chip shares on concerns about the same rumour," said senior vice president at Thanachart Securities, Pichai Lertsupongkit, on Thursday, without mentioning the king directly.

"It's a major concern," another analyst with a Bangkok-based brokerage told AFP, asking not to be named.

"The king is a key institution for political stability in Thailand so the market is closely watching his health," he added.

King Bhumibol is a constitutional monarch but has projected an image as the only unifying figure in the country throughout his 63-year reign, and he is revered as a demigod by many Thais.

Thousands have visited Bangkok's Siriraj Hospital since he was admitted to sign a book for well-wishers, according to officials.

His health is a highly sensitive topic in Thailand, which has been rocked by protests since prime minister Thaksin Shinawatra was toppled in a coup in 2006.

The Bangkok-based analyst said another factor affecting the market was mass rallies planned by "Red Shirt" demonstrators loyal to Thaksin, set to take place on October 17 and 24 in the Thai capital.